Evaluation of the intrinsic conservatism in the design codes for subsea mudmats

Rectangular mudmat foundations for subsea structures are commonly designed according to ISO and API guidelines. However, the modern trend towards deeper waters and heavier structures is currently challenging traditional approaches to foundation design, especially in presence of particularly poor soil conditions. The intrinsic conservatism carried by design codes may not only lead to higher fabrication/installation costs, but also conflict w ith the operational limits of existing installation vessels. Based on the analysis of four design cases, it is argued that ISO/API safety factors may be from 1.5 to 2.3 times as low as the predictions of elastoplastic finite element (FE) simulations. Such a conservatism is also found to considerably increase with the embedment depth, whereas the influence of the foundation aspect ratio is nearly negligible. A simple calculation example shows that the systematic use of numerical analysis in design may result in a 47% reduction of the total foundation area, and an overall decrease in fabrication costs of approximately 16%.